FAC 



F AL 



EXTREME AND MEAN RATIO. 

 A straight line is said to be divided in 

 extreme and mean ratio, when the whole 

 is to the greater part as the greater part 

 to the less ; or when the rectangle con- 

 tained by the whole line and the smaller 

 segment is equal to the square of the 

 greater segment. 



EXTRE'MES {extremus, last). In 

 Logic, the subject and predicate of a pro- 

 position are called its extremes or terms, 

 being, as it were, the two boundaries, 

 having the copula (in regular order) 

 placed between them. In speaking of a 

 syllogism, the word is often understood 

 to imply the extremes of the conclusion. 



EXTRORSE {extrorsum, towards the 

 outside). Turned outwards ; turned away 

 from the axis to which it belongs; a 



term applied, in botany, to those anthers, 

 whose line of dehiscence is towards the 

 petals. Brown uses the term posticce in 

 this sense. 



EXU'VIiE {exuo, to put off). The 

 slough, or cast-off covering of certain 

 animals, as the skins of the serpent and 

 the caterpillar. This term relates, how- 

 ever, in geological language, not only to 

 the rejected envelopes of animals, but to 

 fossil shells and other remains which 

 have been left by animals in the strata of 

 the earth. 



EXUVIATION {exuvice, a slough, the 

 cast-off skin of certain animals). A term 

 applied, in Zoology, to the process by 

 which the crustaceous animals throw off 

 their old shell, and form a new one. 



F 



FA'CET. A term derived from the 

 French, denoting a flat surface, with a 

 definite boundary. The English term 

 face has the same meaning in Geometry, 

 and is applied to the plane which forms 

 one of the surfaces of a polyhedron. 



FACIAL ANGLE. This angle is mea- 

 sured by drawing a line from the pro- 

 minent centre of the forehead to the most 

 advanced part of the lower jaw-bone, and 

 observing the angle which it makes with 

 the horizontal line. By measurement of 

 this angle it has been affirmed that a 

 scale might be traced from " apes with 

 foreheads villainous low " to the African 

 variety of the human species, and from 

 that to the European. 



FACTITIOUS {factito, to practise). 

 Made by art, as factitious cinnabar, in 

 distinction from the natural production. 

 This term is also applied to waters pre- 

 pared in imitation of natural waters, as 

 those of Brighton. 



FA'CTOR. A term applied in Algebra 

 to each of the quantities which are mul- 

 tiplied into one another in order to form 

 ifacere) a product, i. e., to the multiplier 

 and the multiplicand ; thus a and a + x 

 are the factors of the product a (a + x), 

 or a* + ax Factors are also called divi- 

 sors, especially in speaking of a number, 

 which is regarded as the product of seve- 

 ral others. 



Factorial expression. A term some- 

 times applied to an expression of which 

 133 



the factors are in arithmetical progression, 

 as — 



{X + 1) (ar + 2) (X -1- 3) (a: + 4). 



FA'CULA (dim. o{ fax, a torch). A 

 little torch. The term facula is applied 

 to certain luminous spots on the surface 

 of the sun ; they are of irregular form, 

 of variable duration, and are commonly 

 surrounded by a penumbra. By such 

 spots the diurnal revolutions of the 

 planets, as well as that of the sun, have 

 been determined. 



FAHLERZE. Fahlore. A grey copper 

 ore, comprising the arsenical and the 

 antimonial varieties. In this, and in seve- 

 ral other double sulphurets of silver and 

 other metals, the sulphuret of silver and 

 the sub-sulphuret of copper, being iso- 

 morphous, replace each other in inde- 

 terminate proportions. 



FA'HLUNITE, Automalite. A sub- 

 species of octohedral corundum, found in 

 a talcose rock, at Fahlun in Sweden. 



FA'LCATE (Jalx, a scythe or sickle). 

 Sickle-shaped ; any thing plane and 

 curved, with parallel edges. The moon 

 is said to be falcate, when the enlight- 

 ened part appears in the form of a cres- 

 cent. In Zoology, a part is said to be 

 falcate, when it is curved with the apex 

 acute. 



FALCO'NID^. The Falcon tribe ; a 

 family of the Raptores, or Rapacious 

 birds, distinguished from the Vultures by 

 their shorter, sharper, and notched bill. 



